Technical Field
The present invention relates to a method and an apparatus for generating an optical polar return-to-zero (RZ) amplitude modulation signal using a reflective semiconductor optical amplifier, and a wavelength-division-multiplexed passive optical network system using the same. Particularly, the present invention relates to a method and an apparatus for operating a reflective semiconductor optical amplifier whose modulation bandwidth is limited at ultrahigh speed using a polar RZ modulation method.
Background Art
Recently, as the demand for various data services including the Internet is constantly on an increase, a demand for ultrahigh speed optical network is continuously on an increase. As a means to cope with this demand, a wavelength-division-multiplexed passive optical network (WDM PON) is drawing attention. However, since the WDM PON structurally requires a light source having an individual wavelength for each subscriber, it has not been widely used due to the complexity in managing the network and economic issues, etc. In order to solve these problems, a method centering the light source of a specific wavelength on the central office, and installing a colorless light source that can operate at any wavelength at the subscriber end has been suggested. In this case, since all light sources installed at the subscriber end can be implemented as a light source of the same type, the complexity in operating the WDM PON can be relieved, and the light source at the subscriber end can be implemented economically. Recently, a reflective semiconductor optical amplifier (RSOA) is taking center stage as a colorless light source. The RSOA-based wavelength-division-multiplexed passive optical network has a structure transmitting a seed light of a specific wavelength from the central office (CO) to the RSOA located in each optical network unit (ONU), directly modulating the seed light based on a signal using the RSOA at the subscriber end to form an upstream signal, and then transmitting it to the optical receiver located in the central office.
However, since the modulation bandwidth of the RSOA is generally limited to 3 GHz or below, there is a lot of difficulty in increasing the transmission speed per channel of WDM PON to a level of 10 Gb/s or above.
In order for ultrahigh speed operation of the RSOA-based WDM PON, a method for generating a quadrature phase shift keying (QPSK) signal by directly modulating a seed light based on a signal using RSOA and receiving this with the coherent receiver has been reported.
However, since the coherent receiver is too expensive to be applied to an optical access network where cost-effectiveness is important, in order to improve the cost-effectiveness of the network, preferably, the RSOA transmits signals using intensity information, and the optical receiver located at the CO uses a direct detection method.
However, the system using this direct detection method still has disadvantages that the transmission speed per channel is limited by the limited modulation bandwidth of RSOA. For example, a method for increasing the transmission speed per channel of RSOA-based WDM PON, which uses a multi-level amplitude modulation method such as the existing 4-ary pulse amplitude modulation (4-PAM), has been reported, but even in this method, the transmission speed per channel is restricted to 10 Gb/s or below. Thus, in order to provide an ultrahigh speed service of at least 10 Gb/s to each subscriber, and implement an cost-effective RSOA-based WDM PON using the direct detection method, it is necessary to develop a new amplitude modulation method.